This invention relates to recovery of carbonaceous materials from underground deposits. More specifically, this invention relates to the surface and subsurface combustion or retorting of hydrocarbonaceous materials, and attendant product recovery.
Numerous hydrocarbonaceous materials are found in underground deposits; for example crude oil, coal, oil shale tar sands, and others. One method of recovering energy or hydrocarbon from such underground deposits is by combustion. An oxidizing gas such as air, sometimes in conjunction with diluents such as steam, can be provided to an underground combustion or retorting zone so as to combust a portion of the combustible material contained therein and either free hydrocarbon or thereby form materials which are suitable for energy recovery. For example, oxygen or air, and possibly steam, can be passed into a coal deposit so as to form off-gases having combustible materials such as light hydrocarbons and carbon monoxide. These gases can then be combusted directly for heat or energy recovered such as through power generation. Underground combustion can be used in the recovery of petroleum crude oil from certain types of deposits. Air or oxygen, and steam, is passed into an underground deposit and combustion initiated so hot combustion gases will aid in the recovery of such crude oil. Similar technique can be used in the recovery of oil from tar sands. One important use of underground combustion is in the recovery of oil from oil shale.
The term "oil shale" refers to sedimentary deposits containing organic materials which can be converted to shale oil. Oil shale can be found in various places throughout the world, especially in the United States in Colorado, Utah, and Wyoming. Some especially important deposits can be found in the Green River formation in the Piceance Basin, Garfield and Rio Blanco countries, and Northwestern Colorado.
Oil shale contains organic material called kerogen which is a solid carbonaceous material from which shale oil can be produced. Commonly oil shale deposits have variable richness or kerogen content, the oil shale generally being stratified in horizontal layers. Upon heating oil shale to a sufficient temperature, kerogen is decomposed and a liquid is formed. Oil shale can be retorted to form a hydrocarbon liquid either by in situ or surface retorting. In surface retorting, oil shale is mined from the ground, brought to the surface, and placed in vessels where it is contacted with hot material, such as hot shale or gases for heat transfer. Hot retorting temperatures cause shale oil to be freed from the rock. Spent retorted oil shale which has been depleted in kerogen is removed from the reactor and discarded. Some well-known methods of surface retorting are the Tosco, Lurgi, and Paraho processes.
In the Tosco process ceramic balls heated by combustion of retort off-gas, contact shale in a horizontal rotary kiln. Kerogen is broken down and emanates from the kiln as gases which are fractionated to yield liquid products plus off-gas which is in turn combusted to heat the ceramic balls. Spent shale is separated from the ceramic balls by screening, cooled and sent to disposal. The ceramic balls are recycled to a heater.
In the Lurgi process carbon on spent shale is combusted in a riser heater. The hot spent shale is separated from combustion products and mixed with fresh shale feed in a sealed screw conveyor. Gases from this contact are fractionated to yield liquid products and combustible off-gas for use.
In the Paraho process fresh shale is fed to the top of a vertical shaft kiln, contacted with hot gases produced by either in situ combustion of coke on spent shale or externally heated recycle gas. Kerogen breakdown products are withdrawn from the kiln by vapor-collecting tubes near the top of the kiln. Spent shale is removed from the bottom of the kiln by a grate system. Vapors leaving the kiln are separated to yield oil product and combustible gas for use.
Another method of retorting oil shale is the in situ process. In situ retorting of oil shale generally comprises forming a retort or retorting zone underground, preferably within the oil shale zone. The retorting zone can be formed by mining an access tunnel to or near the retorting zone and then removing a portion of the oil shale deposit by conventional mining techniques. About 2 to about 40 percent, preferably about 15 to about 25 percent, of the oil shale in the retorting area is removed to provide void space in the retorting area. The oil shale in the retorting area is then rubblized by well-known mining techniques to provide a retort containing rubblized shale for retorting.
A common method for forming the underground retort is to undercut the deposit to be retorted and remove a portion of the deposit to provide void space. Explosives then are placed in the overlying or surrounding oil shale. These explosives are used to rubblize the shale, preferably forming an area of rubble having uniform particle size and a uniform distribution of gas channel sizes. Generally, the more shale removed, the better the chance of getting a uniform distribution in the rubblized shale, and also the more expensive the mining, hauling and surface retorting operations. Uniform distribution of the rubblized mass improves gas flow there through, and promotes even flame front advance and minimizes the likelihood of the flame front prematurely breaking through at one point of the bottom of the rubblized mass. Premature breakthrough can decrease the total oil yield from a retorting zone. Some of the techniques used for forming the rubblized area are room and pillar mining, sublevel caving, and the like. Because of the stratification of oil shale it is sometimes desirable to selectively mine material based on its mineral or kerogen content for removal from the retorting zone. Also because of stratification, the retorting zone may contain lean oil shale, or rock containing essentially no kerogen.
After the underground retort is formed, the mass of rubblized shale is subjected to retorting. Hot retorting gases are passed through the rubblized shale to effectively form and remove liquid hydrocarbon from the oil shale. This is commonly done by passing a gas such as air or air mixed with steam and/or hydrocarbons through the deposit. Most commonly, air is forced into one end of the retort and a fire or flame front initiated by the use of a burner or the addition of hydrocarbon such as natural gas, propane, and the like. Combustion is then maintained by the burning of coke on spent or partially spent oil shale, thereby producing hot off-gases suitable for retorting. This flame front is then passed slowly through the rubblized deposit to effect the retorting. Not only is shale oil effectively produced, but also a mixture of off-gases from the retorting is also formed. These gases contain hydrogen, carbon monoxide, ammonia, carbon dioxide, hydrogen sulfide, carbonyl sulfide, oxides of sulfur and nitrogen, and low molecular weight hydrocarbons. Generally a mixture of off-gases, water and shale oil are recovered from the retort. This mixture undergoes preliminary separation commonly by gravity to separate the gases from the liquid oil from the liquid water. A product recovery system is provided to separate retort products or by-products. Most commonly, the bottom of underground retorts is in fluid communication with a separation zone, generally at a lower level so that liquids can be transported by gravity flow. Fluid communication is generally provided by mined slots while the main separation zone is commonly a mined room to provide residence time for oil/water/gas separation.
A number of patents describe methods of in situ retorting of oil shale, such as Karrick, L. C., U.S. Pat. No. 1,913,395; Karrick, S. N. U.S. Pat. No. 1,919,636; Uren, U.S. Pat. No. 2,481,051; Van Poollen, U.S. Pat. No. 3,001,776; Ellington, U.S. Pat. No. 3,586,337; Prats, U.S. Pat. No. 3,434,757; Garrett, U.S. Pat. No. 3,661,423; Ridley, U.S. Pat. No. 3,951,456; and Lewis, U.S. Pat. No. 4,017,119 which are hereby incorporated by reference and made a part hereof.
Generally in forming underground in situ retorts, a portion of the formation is mined out and brought to the surface. In the formation of subterranean oil shale retorts, commonly about 2 to about 40, preferably about 15 to about 25 percent of the volume to be retorted is removed and brought to the surface. Because this material contains substantial amounts of recoverable hydrocarbon, it is preferable to retort such material above ground. As discussed previously, it can be done in large reactors such as the Tosco, Lurgi, or Paraho reactors. Another method of retorting such mine deposit is taught in Garrett, which teaches the piling of such material in an area and covering it with a mantle so as to contain gases and passing a hot retorting fluid through such pile to effectively retort the deposit. It generally requires a substantial capital investment to manufacture the large reactor vessels needed for the Tosco, Lurgi, and Paraho processes. It is also relatively expensive to haul the mined material to be retorted in a distant zone and to haul away and dispose spent shale. Generally, the aboveground retorts have a recovery system for the shale oil formed by retorting, and attendant by-products such as water, dust, and off-gases. This seems to be a duplication of the separation system already available underground. In some cases, waste heat present in the off-gases of surface retorting is not effectively used, and therefore reduces the ultimate total energy recovery from a given shale deposit.
It is an object of this invention to provide an improved process for the recovery of hydrocarbon from underground deposits.
It is an object of this invention to provide an underground retort and a substantially surface retort which will have a common recovery zone thereby reducing capital expense and providing a simple method for the recovery of energy from an underground deposit.
It is also an object of this invention to provide an improved design for oil shale retorts. By providing a less expensive method of removing and retorting mined shale, it is practical to remove a larger percentage of shale and thus obtain a rubblized retort with a more uniform distribution of gas channel sizes, and thus minimize flame front breakthrough and obtain higher oil recovery.
It is also an object of this invention to provide a method of retorting oil shale which minimizes the amount of haulage of rubblized matter and of spent shale.